In a letter to FBI Director James Comey on Friday, McCaskill – the top Democrat on the Senate Homeland Security and Governmental Affairs Committee – voiced concern over reports that Flynn discussed U.S. sanctions against Russia with Moscow's ambassador to the U.S. Sergey Kislyak in the month before Trump's inauguration.

“As part of these communications, General Flynn may have struck an agreement or implied future cooperation with Russia and President Vladimir Putin regarding sanctions relief or some form of preferential treatment,” McCaskill said in the letter.

McCaskill said that possibility justifies the committee receiving a closed briefing on the communications.

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“Given the significant implications that any such communications may have had for the nation’s homeland and national security, I request that you provide a closed briefing for the Committee as soon as possible regarding the scope and status of any current FBI investigation related to General Flynn’s contacts with the Russian government,” she said.

Flynn had been in contact with Kislyak a number of times before Trump's electoral win in November. Those contacts have been reviewed by the FBI, who intercepted the communications as part of routine monitoring of foreign officials in the U.S., according to The Washington Post.

So far, FBI officials have not found any evidence suggesting that Flynn promised a Trump administration review of sanctions to Kislyak, the newspaper reported Thursday, citing several sources.

Schiff argued that if Flynn and the Trump administration misled the American people, "his conduct would be all the more pernicious, and he should no longer serve in this Administration or any other."

The Obama administration levied new sanctions against Russia in December amid revelations that the Kremlin had interfered in the 2016 presidential election in favor of Trump. Trump has questioned those allegations, and has accused the U.S. intelligence community of playing politics.

Trump has expressed admiration for Putin, at one point calling him a stronger leader than Obama, despite bipartisan pressure to take a tougher stance on Russia.